COMPLEXITY OF DISSOLVED SUBSTANCES 39
(that is, while the solution is still dilute) since the effect of com-
bination only becomes pronounced in concentrated solution.
Quite a number of examples may be quoted; indeed, this type
seems by far the commonest. Lithium iodide in amyl alcohol,
from which, at lower temperatures Lil, 4C5H12O can be isolated,
and potassium iodide in liquid sulphur dioxide, where combina-
tion to form KI.4SO2 occurs, may serve as examples. C repre-
sents grams of solute per 100 grams of solvent
Lithium Iodide in Amyl Alcohol.1 m Potassium Iodide in Sulphur Dioxide.121
C. M.W. C. M.W.
1-777 I54'4 0-371 282
4-125 159-2 0735 331
8-053 151*2 1-13 352
H'97 151*5 1*58 288
2*05 289
2*46 222
Other examples are furnished by lithium bromide and iodide
in butyl alcohol and lithium bromide in amyl alcohol, where
compounds of the type LiBr, 4ROH are formed ;11S by potassium
iodide in iodine,122 and probably a variety of salts in/-toluidine.105
The possibility of the two other cases follows from the three
already given, so that it is quite possible to have (4) the mole-
cular weight remaining constant over a certain range of concen-
tration, the increase of association being counterbalanced by the
effect of combination, and (5) the molecular weight falling to a
minimum value.
The fact to be borne in mind is that combination in solu-
tion has the effect of reducing the molecular weight. Further
evidence, beyond that given, is that where combination does not
take place, the molecular weight of an associated substance
increases steadily with the concentration, as with lithium nitrate
in ethyl, £r0-butyl, and zso-amyl alcohols, where no compounds
are formed.118
II. The Physical Influence of the Solvent.
A. Separation of the Solute with the Solvent.—This is a factor
which is apt to cause confusion, when not understood, and to lead
to the idea either that a non-associated substance is associated
or that the degree of association is greater than is actually the
case. For if the solute separates in any manner with the solvent,
the solution remaining is more dilute than if no separation oc-
curred. In consequence, the osmotic pressure, as measured by